Monolithic catalysts and related process for manufacture

ABSTRACT

The present invention is directed to a substrate, such as a honeycomb having a plurality of parallel channels defined by the honeycomb walls. The honeycomb has different zones along the length of the channels. The zones are defined by their coating (or lack of coating) and extend for a length of the channel in which there is the same coating and architecture. Soluble components in coating compositions are fixed in their respective zones.

CROSS REFERENCE TO RELATED APPLICATION

[0001] The present application is a continuation-in-part of concurrentlyfiled U.S. Ser. No. 08/______ filed ______, entitled, “METHOD FOR DRYINGA COATED SUBSTRATE”, attorney docket number 3924, and U.S. patentapplication, Ser. No. 08/______ which is a continuation of U.S. Ser. No.08/668,385 filed Jun. 21, 1996, attorney docket number 3983, both hereinincorporated by reference.

FIELD OF THE INVENTION

[0002] The present invention is directed to a vacuum infusion method forcoating a substrate having a plurality of channels such as a monolithicsubstrate used in catalytic convertors.

BACKGROUND OF THE INVENTION

[0003] Catalytic convertors are well known for the removal and/orconversion of the harmful components of exhaust gases. Catalyticconvertors have a variety of constructions for this purpose. In one formthe converter comprises the rigid skeletal monolithic substrate on whichthere is a catalytic coating. The monolith has a honeycomb-typestructure which has a multiplicity of longitudinal channels, typicallyin parallel, to provide a catalytically coated body having a highsurface area.

[0004] The rigid, monolithic substrate can fabricated from ceramics andother materials. Such materials and their construction are described,for example, in U.S. Pat. Nos. 3,331,787 and 3,565,830 each of which isincorporated herein by reference. Alternatively, the monoliths can befabricated from metal foil.

[0005] The monolithic substrate and particularly the multiplicity ofchannels can be coated with a slurry of a catalytic and/or absorbentmaterial.

[0006] One method of coating a prefabricated monolithic substrate is topump the catalyst slurry into the respective channels and then subjectthe coated substrate to a drying operation. Such systems have beenunsuccessful in providing a uniform coating thickness and a uniformcoating profile wherein the catalyst coating is deposited over the samelength of each of the channels.

[0007] It has been proposed to employ a vacuum to draw the catalystslurry upwardly through the channels. For example, Peter D. Young, U.S.Pat. No. 4,384,014 discloses the creation of a vacuum over themonolithic substrate to remove air from the channels and then drawingthe catalyst slurry upwardly through the channels. The vacuum is thenbroken and excess slurry is removed, preferably by gravity drainage.

[0008] James R. Reed, et al., U.S. Pat. No. 4,191,126, discloses thedipping of the monolithic substrate into a slurry and then utilizingsubatmospheric pressure to purge the excess coating slurry from thesurfaces of the support. The applied vacuum is intended to unplug thechannels so that the slurry is drawn over the surfaces of each of thechannels.

[0009] An improvement in these systems is disclosed in Thomas Shimrock,et al., U.S. Pat. No. 4,609,563, incorporated herein by reference. Thissystem encompasses a method of vacuum coating ceramic substrate memberswith a slurry of refractory and/or catalyst metal components whereinprecisely controlled, predetermined amounts of the slurry are meteredfor application to the ceramic monolithic substrate. The monolithicsubstrate is lowered into a vessel, also known as a dip pan, ofpreferably predetermined dimensions to a predetermined depth containingthe precise amount of slurry which is to be coated onto the substrate.The slurry is then drawn up by a vacuum which is applied to the end ofthe substrate opposite to the end which is immersed in the bath. Nodraining or purging of excess coating slurry from the substrate isnecessary nor is any pre-vacuum application step required to eliminateair.

[0010] A further improved method is disclosed in U.S. Ser. No.08/962,363, filed Oct. 31, 1997 which is a continuation of 08/668,385filed Jun. 21, 1996 and entitled, “METHOD FOR COATING A SUBSTRATE”.There is disclosed a vacuum infusion method for coating monolithicsubstrates in which each of the channels comprising the substrate iscoated with the same thickness of the coating and is characterized by auniform coating profile. The term “uniform coating profile” as usedherein means that each channel of the substrate will be coated over thesame length. In particular, the method is directed to a vacuum infusionmethod for coating a substrate having a plurality of channels with acoating media comprising:

[0011] a) partially immersing the substrate into a vessel containing abath of the coating media, said vessel containing an amount of coatingmedia sufficient to coat the substrate to a desired level withoutreducing the level of the coating media within the vessel to below thelevel of the immersed substrate;

[0012] b) applying a vacuum to the partially immersed substrate at anintensity and a time sufficient to draw the coating media upwardly fromthe bath into each of the channels to form a uniform coating profiletherein; and

[0013] c) removing the substrate from the bath.

[0014] Optionally, after the coating media is applied to the substrateand as the substrate is being removed from the bath, a vacuum continuesto be applied to the substrate at an intensity equal to or greater thanthe intensity of the vacuum imposed on the partially immersed substrate.

[0015] The above referenced Parent U.S. Ser. No. 08/668,385 (attorneydocket number 3983) discloses that a substrate may be inverted andcoated from an opposite end producing two coatings having uniformcoating profile. There is disclosed that if there is any overlap, it ismuch smaller than with prior art methods.

[0016] Copending U.S. Ser. No. 08/______ (attorney docket number 3924)discloses that after coating, the substrate or monolithic honeycomb canbe rapidly and thoroughly dried without adversely affecting the coatingprofile. In particular, the disclosed method dries a monolithicsubstrate having a plurality of channels and a coating media thereon byremoving the coated monolithic substrate from a bath containing thecoating media while the coating media is in a wet condition. A vacuum isapplied to the coated monolith substrate at an intensity in timesufficient to draw vapor out of the channels without substantiallychanging the coating profile within the channels. In a specific andpreferred embodiment, the vacuum is imposed at one end of the substratewhile gas at an elevated temperature is introduced into the opposite endof the substrate to facilitate drying.

[0017] Monolithic honeycombs containing different catalyst compositionsin zones along the length of the honeycomb are known for use incatalytic combustion processes from references such as WO 92/09848. Itis disclosed that graded catalyst structures can be made on ceramic andmetal monoliths by a variety of processes. Monoliths can be partiallydipped in washcoat and excess washcoat blown out of the channel. Theprocess is repeated by dipping further into the washcoat sol.Alternatively, catalyst is disclosed to be applied to metal foil whichis then rolled into a spiral structure. The washcoat is disclosed to besprayed or painted onto the metal foil or applied by other knowntechniques such as by chemical vapor deposition, sputtering, etc.

[0018] It is also disclosed in WO 92/09848 that the catalyst can beapplied as a mixture of active catalyst (such as palladium) and a highsurface support (Al₂ O3, ZrO₂,and Si₂O, etc.). These are disclosed to beprepared by impregnating the palladium onto the high surface are oxidepowder, calcining, then converting to a colloidal sol. In a secondmethod, the high surface area wahcoat may be applied first to themonolith or metal foil and fixed in place. Then the catalyst, e.g.,palladium, may be applied by the same dipping or spraying procedure.

[0019] Three-way conversion catalysts (TWC) have utility in a number offields including the treatment of exhaust from internal combustionengines, such as automobile and other gasoline-fueled engines. Emissionsstandards for unburned hydrocarbons, carbon monoxide and nitrogen oxidescontaminants have been set by various governments and must be met, forexample, by new automobiles. In order to meet such standards, catalyticconverters containing a TWC catalyst are located in the exhaust gas lineof internal combustion engines. The catalysts promote the oxidation byoxygen in the exhaust gas of the unburned hydrocarbons and carbonmonoxide and the reduction of nitrogen oxides to nitrogen.

[0020] Known TWC catalysts which exhibit good activity and long lifecomprise one or more platinum group metals (e.g., platinum or palladium,rhodium, ruthenium and iridium) located upon a high surface area,refractory oxide support, e.g., a high surface area alumina coating. Thesupport is carried on a suitable carrier or substrate such as amonolithic carrier comprising a refractory ceramic or metal honeycombstructure, or refractory particles such as spheres or short, extrudedsegments of a suitable refractory material.

[0021] U.S. Pat. No. 4,134,860 relates to the manufacture of catalyststructures. The catalyst composition can contain platinum group metals,base metals, rare earth metals and refractory, such as alumina support.The composition can be deposited on a relatively inert carrier such as ahoneycomb.

[0022] In a moving vehicle, exhaust gas temperatures can reach 1000° C.or higher, and such elevated temperatures cause the activated alumina(or other) support material to undergo thermal degradation caused by aphase transition with accompanying volume shrinkage, especially in thepresence of steam, whereby the catalytic metal becomes occluded in theshrunken support medium with a loss of exposed catalyst surface area anda corresponding decrease in catalytic activity. It is a known expedientin the art to stabilize alumina supports against such thermaldegradation by the use of materials such as zirconia, titania, alkalineearth metal oxides such as baria, calcia or strontia or rare earth metaloxides, such as ceria, lanthana and mixtures of two or more rare earthmetal oxides. For example, see C. D. Keith, et al., U.S. Pat. No.4,171,288. Reference is also made to a review of threeway catalysts inthe Background of U.S. Ser. No. 08/962,283, filed Oct. 31, 1997entitled, “CATALYST COMPOSITION” (attorney docket number 4136A CIP).

[0023] Preferred catalysts and catalyst structures which contain oxygenstorage components are disclosed in WO 95/35152, WO 95/00235 and WO96/17671 hereby incorporated by reference. These references disclosemultiple layer catalysts. The discrete form and second coats ofcatalytic material, conventionally referred to as “washcoats”, can becoated onto a suitable carrier with, preferably, the first coat adheredto the carrier and the second coat overlying and adhering to the firstcoat. With this arrangement, the gas being contacted with the catalyst,e.g., being flowed through the passageways of the catalyticmaterial-coated carrier, will first contact the second or top coat andpass therethrough in order to contact the underlying bottom or firstcoat. However, in an alternative configuration, the second coat need notoverlie the first coat but may be provided on an upstream (as sensed inthe direction of gas flow through the catalyst composition) portion ofthe carrier, with the first coat provided on a downstream portion of thecarrier. Thus, to apply the washcoat in this configuration, an upstreamlongitudinal segment only of the carrier would be dipped into a slurryof the second coat catalytic material, and dried, and the undippeddownstream longitudinal segment of the carrier would then be dipped intoa slurry of the first coat catalytic material and dried.

[0024] There is a need to refine methods and articles to strategicallylocate catalyst on substrates.

SUMMARY OF THE INVENTION

[0025] The present invention is directed to a substrate, preferably ahoneycomb comprising a plurality of channels defined by the honeycombwalls. The channels, and wall elements are parallel and typically axialto the axis of the substrate. The honeycomb has an inlet end and anoutlet end, with at least some of the channels having a correspondinginlet and outlet. There is a first inlet layer located on the walls andextending for at least part of the length from the inlet end toward theoutlet end to an inlet layer axial end. The first inlet layer extendsfor only part of the length from the inlet end toward the outlet end.The at least one inlet layer comprises a first inlet compositioncomprising at least one first inlet component selected from first inletbase metal oxides. The first inlet layer is coated by a methodcomprising the steps of passing a fluid containing the first inletcomposition into the inlet end of the substrate to form the first inletlayer, and then applying a vacuum to the outlet end while forcing aheated gas stream through the channels from the inlet end withoutsignificantly changing the length of the first inlet layer. In certainembodiments a one or more layer can be applied over the entire channellength by conventional methods and used in combination with the methodof the present invention.

[0026] The first inlet base metal oxides can be selected from a firstinlet refractory oxide, a first inlet rare earth metal oxide, a firstinlet transition metal oxide, a first inlet alkaline earth metal oxideand a molecular sieve. Preferably the first inlet composition comprisesat least one first inlet precious metal component.

[0027] In a specific and preferred embodiment there can be at least onesecond inlet layer located on the walls and extending for at least partof the length from the inlet end toward the outlet end to a second layeraxial end. The at least one second layer can be supported directly orindirectly on the first inlet layer for at least part of the length ofthe first inlet layer, the at least one second layer comprising a secondinlet composition comprising at least one second inlet componentselected from second inlet base metal oxides. The at least one secondinlet layer coated by a method comprising the steps of passing a fluidcontaining the at least one second inlet composition into the inlet endof the substrate to form the at least one inlet layer and applying avacuum to the outlet end while forcing a heated gas stream through thechannels from the inlet end without significantly changing the length ofthe at least one second inlet layer.

[0028] The at least one second inlet base metal oxides are selected froma second inlet refractory oxide, a second inlet rare earth metal oxide,a second inlet transition metal oxide, a second inlet alkaline earthmetal oxide, and a molecular sieve. Preferably the second inletcomposition comprises at least one second inlet precious metalcomponent. Preferably there is at least one precious metal componentselected from the first inlet precious metal component and the secondinlet precious metal component. The at least one precious metalcomponent is preferably selected from the first inlet precious metalcomponent and the second inlet precious metal component and saidprecious metal components are selected from at least one of platinum,palladium, rhodium, and iridium components.

[0029] In another specific embodiment there can be a first outlet layerlocated on the walls and extending for at least part of the length fromthe outlet end toward the inlet end to an outlet layer axial end. Thefirst outlet layer extends for only part of the length from the outletend toward the inlet end. The at least one outlet layer comprises afirst outlet composition comprising at least one first outlet componentselected from first outlet base metal oxides. The first outlet layer iscoated by a method comprises the steps of passing a fluid containing thefirst outlet composition into the outlet end of the substrate to formthe first outlet layer and applying a vacuum to the outlet end whileforcing a heated gas stream through the channels from the outlet endwithout significantly changing the length of the first outlet layer.

[0030] The first outlet base metal oxides are selected from a firstoutlet refractory oxide, a first outlet rare earth metal oxide, a firstoutlet transition metal oxide, a first outlet alkaline earth metaloxide, and a molecular sieve. Preferably the first outlet compositioncomprises at least one first outlet precious metal component.

[0031] Another embodiment comprises at least one second outlet layerlocated on the walls and extending for at least part of the length fromthe outlet end toward the inlet end to a second layer axial end. The atleast one second layer can be supported directly or indirectly on thefirst outlet layer for at least part of the length of the first outletlayer. The at least one second layer comprising a second outletcomposition comprising at least one second outlet component selectedfrom second outlet base metal oxides. The at least one second outletlayer coated by a method comprising the steps of passing a fluidcontaining the at least one second outlet composition into the outletend of the substrate to form the at least one outlet layer, and thenapplying a vacuum to the outlet end while forcing a heated gas streamthrough the channels from the outlet end without significantly changingthe length of the at least one second outlet layer. The at least onesecond outlet base metal oxides are selected from a second outletrefractory oxide, a second outlet rare earth metal oxide, a secondoutlet transition metal oxide, a second outlet alkaline earth metaloxide, and a molecular sieve. Preferably the second outlet compositioncomprises at least one second outlet precious metal component.Preferably there is at least one precious metal component selected fromthe first outlet precious metal component and the second outlet preciousmetal component and said precious metal components are selected from atleast one of platinum, palladium, rhodium, ruthenium and iridiumcomponents. In each of the embodiments, for the various layers includingthe first layer and at least one second inlet layer, and the first layerand at least one second outlet layer the heated gas is preferably airbut can be any suitable gas such as nitrogen. The temperature of theheated gas is preferably from about 75° C. to about 400° C. Thetemperature of the heated gas is preferably from 75° C. to 200° C. todry the various layers. The temperature of the heated gas is preferablyfrom 200° C. to 400° C. to fix the precious metal component of thevarious layers. The heated gas is passed over the layers for asufficient time to dry as to fix the precious metal of compositions ofthe various layers. The at least one precious metal component selectedfrom the first outlet precious metal component and the second outletprecious metal component.

[0032] Structurally, the architecture of the layers can vary as desired.For example at least a portion of at least one of the first or secondinlet layers over laps with at least one of the first or second outletlayers. A zone can also have a continuous gradient of materialconcentration versus layer thickness. Preferably the substrate has atleast two adjacent zones, a first zone and a second zone, each extendingaxially along the length of conduit. The first zone can extend from theinlet and the second or outlet zone extends from the outlet along aseparate length of the conduit than the first zone with each zonecomprising the same catalyst architecture within said zone. The adjacentzones have different compositions and/or architecture. In a specificembodiment at least one layer of said first zone, and at least one layerof said second zone overlap to form at least one intermediate zonebetween the first zone and the second zone. There can be at least threezones, or there can be an uncoated zone between the first zone and thesecond zone.

[0033] The substrate can comprise a monolithic honeycomb comprising aplurality of parallel channels extending from the inlet to the outlet.The monolith can be selected from the group of ceramic monoliths andmetallic monoliths. The honeycomb can be selected from the groupcomprising flow through monoliths and wall flow monoliths. In specificembodiments the composition of the layers can include the recitedprecious metals. At least one layer can contain no precious metalcomponent. A preferred article comprises at least one inlet layer and atleast one outlet layer. The at least one inlet composition comprises atleast one first inlet refractory oxide composition or compositecomprising a first inlet refractory oxide selected from the groupconsisting of alumina, titania, zirconia and silica, a first inlet rareearth metal oxide and at least one first inlet precious metal component.The at least one outlet layer comprises an outlet composition comprisesat least one outlet refractory oxide composition or composite comprisingan outlet refractory oxide selected from the group consisting ofalumina, titania, zirconia and silica, an outlet rare earth metal oxideand at least one outlet precious metal component.

[0034] In a specific embodiment the inlet compositions containsubstantially no oxygen storage components. More specifically the inletcompositions contain substantially no oxygen storage components selectedfrom praseodymium and cerium components. In specific embodiments atleast one of the outlet compositions contain an oxygen storagecomponents. More specifically at least one of the outlet compositionscontains oxygen storage components selected from praseodymium and ceriumcomponents. Preferably at least one inlet precious metal component isfixed to the at least one of the at least one inlet refractory oxidecomposition or composite and the first rare earth metal oxide, and theat least one outlet precious metal component is fixed the at least oneof the at least one outlet refractory oxide composition or composite andthe rare earth metal oxide. The present invention includes a methodcomprising passing at least one inlet end fluid comprising an inlet endcoating composition into a substrate as recited above. For the purposeof the present invention a fluid includes liquids, slurries, solutions,suspensions and the like. The aqueous liquid passes into the channelinlets and extending for at least part of the length from the inlet endtoward the outlet end to form at least one inlet end layer coating, withat least one inlet end coating extending for only part of the lengthfrom the inlet end toward the outlet end. A vacuum is applied to theoutlet end while forcing a gas stream through the channels from theinlet end after the formation of each inlet end coating withoutsignificantly changing the length of each inlet layer coating. At leastone outlet end aqueous fluid comprising at least one outlet end coatingcomposition is passed into the substrate through the at least some ofthe channel outlets at the substrate outlet end. The aqueous liquidpasses into the channels and extending for at least part of the lengthfrom the outlet end toward the inlet end to form at least one outlet endlayer coating. The method can further comprise applying a vacuum to theinlet end while forcing a gas stream through the channels from theoutlet end after the formation of each outlet end coating withoutsignificantly changing the length of each outlet layer coating.

[0035] The method can further comprise the step of fixing the at leastone precious metal component selected from the inlet precious metalcomponent of the at least one inlet layer and the outlet precious metalcomponent of the at least one outlet layer to said at least one of therespective inlet or outlet component selected from the inlet refractoryoxide and inlet rare earth metal oxide components and the outletrefractory oxide and outlet rare earth metal oxide components. Thefixing can be conducted prior to coating the inlet and outlet layers.The step of fixing can comprise chemically fixing the precious metalcomponent on the respective refractory oxide and/or rare earth metaloxide. Alternatively, the step of fixing can comprise thermally treatingthe precious metal component on the respective refractory oxide and/orrare earth metal oxide. The step of fixing comprises calcining theprecious metal component on the respective refractory oxide and/or rareearth metal oxide. The step of calcining can be conducted at from 200°C., preferably 250° C. to 900° C. at from 0.1 to 10 hours. The steps ofthermally fixing each layer are preferably conducted after coating andprior to coating a subsequent layer. The step of thermally treating thesubstrate upon completion of coating all layers at from 200° C. to 400°C. at from 1 to 10 seconds. The steps of calcining is preferably thesubstrate conducted upon completion of coating all layers. The step ofcalcining is conducted at from 250° C. to 900° C. at from 0.1 to 10hours.

[0036] The honeycomb has different zones along the length of thechannels. The wall in the different zones can be uncoated or coated withdifferent catalyst compositions or architectures. The term“architecture” is used to mean the physical design of the coating in azone considering parameters such as the number of layers of coatingcompositions, the thickness of the layers, and the order of layers wherethere are more than one layer. The zones are defined by their coating(or lack of coating) and extend for a length of the channel in whichthere is the same coating and architecture. For example, a two layeredcatalyst coating defines a zone until it bounds with an adjacent zonehaving different compositions or different numbers of layers.Nonadjacent zones can have the same architecture and compositions. Anadvancement of the present invention is that soluble components incoating compositions are fixed in their respective zones. For example,precious metal which may be present is fixed in its respective zone andeven layer within a zone. In this way, a single monolithic honeycomb canbe multifunctional with a minimum and preferably no migration ofprecious metal components from zone to zone, particularly during theprocess of manufacture. The terms “fixed” and “segregated” shall meanthat components within a zone, and within a layer within a zone remainwithin the zone with a minimum and preferably no migration or diffusionduring the processing to manufacture the catalyzed substrate. Anadvancement of the monolith of the present invention is that there is aminimum of migration precious metal from one zone to another, even wherea composition from one zone overlaps with the composition in anotherzone.

[0037] The first or inlet zone preferably comprises an inlet compositioncomprising at least one inlet refractory oxide composition or compositecomprising a first refractory oxide selected from the group consistingof alumina, titania, zirconia, silica, an inlet rare earth metal oxide,a molecular sieve such as a zeolite and at least one first preciousmetal component, and the second or outlet zone comprises an outletcomposition comprising at least one outlet refractory oxide compositionor composite comprising an outlet refractory oxide selected from thegroup consisting of alumina, titania, zirconia, and silica, a rare earthmetal oxide, a molecular sieve such as a zeolite and at least one secondprecious metal component. The at least one first precious metalcomponent can be fixed to the at least one of the at least one firstrefractory oxide composition and the first rare earth metal oxide. Theat least one second precious metal component can be fixed to at leastone of the at least one second refractory oxide composition and thesecond rare earth metal oxide. The first precious metal is in the firstlayer segregated from the second layer and the second precious metal isin the second layer segregated from the first layer. Where there is morethan one layer, e.g. sublayers, in a zone, preferably the precious metalin a layer remains segregated within that layer.

[0038] Preferably, the precious metal can be prefixed the supports.Alternatively the method further comprises fixing the soluble componentsin the layer such as at least one precious metal component selected fromthe first precious metal component of the inlet layer and the secondprecious metal component of the outlet layer to said at least one of therespective first or second component selected from the first refractoryoxide and first rare earth metal oxide components, and the secondrefractory oxide and second rare earth metal oxide components, thefixing being conducted prior to coating the inlet and outlet layers. Thestep of fixing can comprises chemically fixing the precious metal on therespective refractory oxide and/or rare earth metal oxide. Morepreferably, the step of fixing comprises thermally treating the preciousmetal on the respective refractory oxide and/or rare earth metal oxide.The step of thermally treating the substrate upon completion of coatingone or more layers at from 200° C. to 400° C. at from 1 to 10 , andpreferably 2 to 6 seconds. The heat is provided by forcing a gas stream,preferably air which is heated to from 200° C. to 400° C. Thistemperature range has been found to substantially fix the solublecomponents such as precious metal components. The combination of flowrate and temperature of the gas stream should be sufficient to heat thecoating layer and preferably, providing a minimum of heat to theunderlying substrate to enable rapid cooling in the subsequent coolingstep prior to application of subsequent layers. Preferably, the steps ofthermally fixing each layer, preferably followed by cooling with ambientair, are conducted after coating and prior to coating a subsequentlayer. The cooling step is preferably conducted using ambient airtypically at from 5° C. to 40° C. at from 2 to 20, and preferably 4 to10 seconds at a suitable flow rate. The combination of the ambient airflow rate and temperature of the gas stream should be sufficient to coolthe coating layer. This method permits continuous coating of a pluralityof layers on a substrate to form the above described article of thepresent invention.

[0039] A preferred method comprises the step of fixing the at least oneprecious metal component selected from the first precious metalcomponent of the first layer and the second precious metal component ofthe second layer to said at least on to the respective first or secondcomponent selected from the first refractory oxide and first rare earthmetal oxide components, and the second refractory oxide and second rareearth metal oxide components, the fixing being conducted prior tocoating the first and second layers.

[0040] In yet another embodiment the method comprises the step ofapplying a vacuum to the partially immersed substrate at an intensityand a time sufficient to draw the coating media upwardly to apredesignated distance from the bath into each of the channels to form auniform coating profile therein for each immersion step. Optionally, andpreferably the substrate can be turned over to repeat the coatingprocess from the opposite end with the second composition. The coatedsubstrate should be thermally fixed after forming the inlet layer, andafter turning the substrate over and forming the outlet layer.

[0041] The method can include a final calcining step. This can beconducted in an oven between coating layers or after the coating of allthe layers on the substrate has been completed. The calcining can beconducted at from 250° C. to 900° C. at from0.1 to 10 hours andpreferably from 450° C. to 750° C. at from at from 0.5 to 2 hours. Afterthe coating of all layers is complete the substrate can be calcined.

BRIEF DESCRIPTION OF THE DRAWINGS

[0042]FIG. 1 is a view of in perspective of a honeycomb substrate.

[0043]FIG. 2 is a sectional view of the honeycomb of FIG. 1 alongSection 2-2.

[0044] FIGS. 3 to 8 a schematic drawings illustrating various examplesubstrate designs of the present invention.

[0045]FIG. 9 is a schematic flow chart illustrating the method of thepresent invention.

[0046]FIG. 10 is a schematic illustration of a motor vehicle containingboth a close coupled catalyst and an under the floor catalyst.

[0047]FIG. 11 is a schematic illustration of a motor vehicle exhaustline containing both a close coupled catalyst and an under the floorcatalyst.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0048] Preferred embodiments of the present invention will be understoodby those skilled in the art by reference to the accompanying Figures.

[0049] As shown in FIG. 1, the present invention is directed to asubstrate, preferably a honeycomb 10 comprising an outer surface 12, aninlet end 14 and an outlet end 14′. There are a plurality of parallelchannels 16 defined by the honeycomb walls 18. Each channel has acorresponding inlet and outlet. The honeycomb 10 has different zonesalong the length of the channels. The walls 18 of the different zonescan be uncoated or coated with different catalyst compositions orarchitectures. The zones are defined by their coating (or lack ofcoating) and extend for a length of the channel in which there is thesame coating and architecture. For example, a two layered catalystcoating defines a zone until it bounds with an adjacent zone havingdifferent compositions or different numbers of layers. Nonadjacent zonescan have the same architecture and compositions. FIG. 2 shows asectional view 2-2 of the honeycomb 10 of FIG. 1 containing three zones,a first zone 20 and a second zone 22 which are coated and a third zone24 which is uncoated and between zones 20 and 22.

[0050] An advancement of the present invention is that solublecomponents in coating compositions are fixed in their respective zones.For example, precious metal which may be present is fixed in itsrespective zone and even layer within a zone. In this way, a singlemonolithic honeycomb can be multifunctional with a minimum andpreferably no migration of precious metal components or other materialshaving aqueous solubility or other diffusion characteristics from zoneto zone, particularly during the process of manufacture. For thepurposes of the present application the components within a zone aresegregated, and preferably within a layer within a zone are alsosegregated in that layer and remain within the zone with a minimum. Mostpreferably there is a minimum of component migration or diffusion duringthe processing to manufacture the substrate. There is a minimum ofmigration precious metal from one zone to another, even where acomposition from one zone overlaps with the composition in another zone.

[0051] FIGS. 3 to 8 illustrate examples of honeycomb 10 with a pluralityof architecture designs. Common elements of FIGS. 3 to 8 and FIGS. 1 and2 have the same reference characters. FIGS. 3 to 8 illustrate a threezone honeycomb 10 showing a variety of coating architecture on walls 18in each of the zones 20, 22 and 24. The embodiments of FIGS. 3, 4, 5, 7and 8 illustrate coating architecture where the coating is fluid, suchas a slurry, which passes into the inlet end 14 and/or outlet end 14′ ofhoneycomb 10. It is recognized that in certain embodiments, such asshown in FIG. 6, internal zones can be coated without having to passinto inlet end 14 or outlet end 14′ such as by application to foil priorto assembly of a metal, paper or polymeric monolith.

[0052]FIG. 3 illustrates an embodiment wherein the first zone 20 iscoated with a single layer 26 of a catalytic composition. The secondzone 22 is likewise coated with a single layer of a catalyst composition28 which may be the same or different than catalyst composition 26. Thethird zone 24 remains uncoated. FIG. 4 illustrates an alternativeembodiment to that of FIG. 3 wherein the first zone 20 comprises atwo-layer catalyst coating having an inner layer 30 which, in turn, iscoated with an outer layer 32. The second zone 22 likewise is shownhaving a two-layer catalytic architecture having inner layer 34 coatedwith outer layer 36. Either zone 20 at the inlet end 14 or zone 22 atthe outlet end 14 prime can have one or more catalytic layers orcatalytic and non-catalytic layers. The third zone 24 is likewise showwithout a catalytic layer. As will be described below, zone 24 can havea non-catalytic or catalytic layers.

[0053]FIG. 5 illustrates an embodiment wherein the first zone 20comprises a catalytic layer 38 which extends into zone 24 as a catalyticouter layer. Likewise, the second zone 22 at the outlet has a catalytic40 which extends into third zone 24 as catalytic inner layer 40 doubleprime.

[0054]FIG. 6 illustrates an embodiment wherein there are three zones,20, 22 and 24. Zones 20 and 22 are shown to have catalytic coatings 42and 44, respectively. Inner zone 24 is illustrated as having a catalyticcoating 46 which can be the same as either catalytic coating 42 orcatalytic coating 44 or be an independent coating which is catalytic ornon-catalytic.

[0055]FIGS. 7 and 8 illustrate gradient embodiments. First zone 20 atthe inlet end 14 contains three layers, inner layer 46, middle layer 48and outer layer 50. Inner layer 46 extends for the complete length ofzones 20, 22 and 24. In second zone 22 at the outlet end layer 46 primeis the only layer. In the third zone 24 there are two layers; innerlayer 46 double prime and outer layer 48 double prime which is anextension of middle layer 48 from the first zone 20. In an alternativeembodiment first zone 20 contains three layers; 52, 54 and 56. Innerlayer 52 extends only through first zone 20. Middle layer 54 extendsinto third zone 24 as inner layer 54 double prime. Outer layer 56 ofzone 20 extends into third zone 24 as outer layer 56 double prime andinto second zone 22 as single layer 56 single prime.

[0056] As will be reviewed below, the coated zone substrates of thepresent invention can be produced by a variety of methods. However, asindicated, the composition of the coatings defining each layer andwithin each zone should be segregated. That is, materials should remainin their respective layers and zones during processing and manufacturingwith a minimum and preferably no migration of components from layer tolayer and zone to zone. This is particularly important with respect tosoluble components such as precious metal salts.

[0057] The coated honeycomb substrate of the preferred embodiments ofthe present invention may be made by a variety of processes. Suchprocesses are preferably directed to the use of a formed monolithichoneycomb such as a ceramic or metallic honeycomb and passing fluid,such as a slurry, composition into the inlet and/or outlet ends to adesired distance to form the respective zones. Alternatively, materialsuseful to form monolithic honeycombs, such as sheets of metal foil,paper or polymeric material, can be coated with catalytic compositionsprior to forming the monolith. Upon forming the monolith, the variouscompositions are located at the desired zones extending from the inlettoward the outlet within the monolith. For examples a coated substratesuch as illustrated in FIG. 6 can be made by a combination of processes.For example, certain zones such as zone 24 can be precoated onto a foiland than the outer zones 20 and 22 can be coated by passing a coatingcomposition into the inlet and outlet of the formed honeycomb. When madeusing coated materials to assemble the substrate, useful methods ofcoating elements such as foil include chemical vapor deposition,sputtering, paint coating, rolling, printing and the like. Reference ismade to International Publication No. WO 92/09848, hereby incorporatedby reference for various methods to make coated honeycombs whereindifferent compositions are located along the length of honeycombchannels.

[0058] As indicated above, it is preferred to the segregate preciousmetal component of the coating composition within various layers andbetween zones. Preferably, the catalytic active material, at least inthe first layer, can be applied prior to fixing onto a refractorysupport material and the substrate containing the layer can be thermallytreated to convert the precious metal salt to an insoluble preciousmetal oxide which would result in a minimum of diffusion of preciousmetal to adjacent layers. Additionally or alternatively the preciousmetal can be fixed onto support particles such as refractory oxidesprior to forming a catalyst slurry composition useful for application ofthe coating. In this way, when the catalyst composition on the coatingis dried, the precious metal would be in oxide form fixed to therefractory oxide support and there would be a minimum of precious metalmigration to layers on which the coating is placed on a substrate orinto subsequent layers placed on top of a given layer.

[0059] A preferred method comprises coating a first zone of a substrate,as recited above, with at least one first layer comprising a firstcomposition. A second zone of the substrate is coated with at least onesecond layer comprising a second composition. Preferably, the method ofthe present invention provides for a continuous production of aplurality of honeycombs of the present invention.

[0060]FIG. 9 is a schematic flow chart illustrating the various steps Athrough E in a specific embodiment of the present invention. Commonelements in FIG. 9 and various other Figures have the same referencecharacters. The method of the present invention is useful for acontinuous production.

[0061] In Step A, honeycombs 10 are continuously fed into an apparatusfor coating. The honeycomb 10 is retained by a suitable retaining meanssuch as clamp 60. The honeycomb 10 may be weighed before coating orotherwise prepared. The honeycomb proceeds from Step A to Step B. InStep B honeycomb 10 is immersed in a vessel such a dip pan 62 having aregion in the form of a reservoir 64 containing a coating media 66. Asuitable means is used to apply a vacuum to the top or outlet end 14′ ofhoneycomb 10. Preferably, hood 68 is sealingly applied to the top oroutlet end 14′ of honeycomb 10 and a vacuum is applied by a suitablevacuum means, such as a vacuum pump (not shown) through conduit 69 tothe top end 14′ of the honeycomb 10 to create a pressure drop andthereby draw the coating media 66 from the reservoir 64 into the bottomor inlet end 14 of the honeycomb 10 so as to coat the channels 16 atleast over a portion of their length. This coating is conducted in themanner disclosed in copending patent application Ser. No. 08/______(attorney docket number 3924) entitled, “METHOD FOR DRYING A COATEDSUBSTRATE”, which is incorporated herein by reference. When the coatingis to be applied for only part of the channel length, there can be alimited amount of fluid (coating media) in the reservoir. When the fluidis all removed it coats a predetermined length and air is sucked intothe channel. The front edge of the fluid which had filled the channelsbreaks and there is an open path from the inlet to the outlet. Thecomposition forms a coating length on the wall up to the predeterminedlength. In Step B, the vacuum applied can be from 5 to 15 and typically5 to 10 inches of water. The coating step takes place from 1 to 10seconds and preferably 2 to 4 seconds.

[0062] The coating applied in Step B is then dried in accordance withStep C. A useful description of the drying step is described in thereferenced patent application Ser. No. 08______ (attorney docket number3924). Step C is an operative engagement of the vacuum apparatus forpulling vapors through the substrate and a blowing device for forcinggas (e.g., heated air) through the substrate in order to dry thecoating. The honeycomb 10 continues to be retained by a suitableretaining means such as clamp 60 during the drying operation. A suitablemeans is used to apply a vacuum to the top or outlet end 14′ ofhoneycomb 10. Preferably, hood 68 can continue to be applied or a newhood 70 is sealingly applied to the top or outlet end 14′ of honeycomb10 and a vacuum is applied by a suitable vacuum means, such as a vacuumpump (not shown) through conduit 72 to the top end or outlet end 14′ ofthe honeycomb 10. There is a means for forcing or pushing a gas (e.g.,hot air) into the channels 16 of the honeycomb. The apparatus includes ahood 76 which has means to be sealingly applied to the bottom or inletend 14 of honeycomb 10.

[0063] In the operation of Step C, a vacuum is generated by a suitablevacuum generating device to draw gas from the top or outlet end 14′through conduit 72. A blower (not shown) or suitable device is activatedto force a hot gas into conduit 78 and into the bottom or inlet end 14of honeycomb 10. Accordingly, vapors are drawn from the honeycomb 10outlet 14′ through hood 70 and out conduit 72, while hot air is forcedupwardly through conduit 78 into the hood 76 and up into the bottom orinlet end 14 of honeycomb 10. As a consequence, vapors within thechannels 16 of the honeycomb 10 are drawn out of the channels and hotgas is forced through the channels of honeycomb 14 to dry the coating.

[0064] The intensity of the vacuum imposed during the drying step canvary depending upon the cross-sectional areas of the channels 16, thecomposition and thickness of the coating media applied to each channel.Generally, the intensity of the vacuum will be in the range of fromabout 5 to about 15 inches of water. A device for imposing a vacuum canbe, for example, a Paxton Blower. The hot blowing gas system can be inthe form of jet air kerosene heater having a heating capacity of, forexample, about 50,000 BTU. In operation, once the substrate is removedfrom the reservoir of the coating media in Step B, the vacuum draws thevaporized constituents from the channels at a vacuum of from about 5 to15 inches of water, for typically from 2 to 40 seconds, preferably 2 to10 seconds, and most preferably 2 to 6 seconds. The vacuum is maintaineduntil the vapors are dissipated. During or after imposition of thevacuum, the hot gas generating system can generate a hot gas (e.g., hotair) at a suitable temperature (e.g., from about 75° to 400° C., mosttypically from 75° to 200° C.) and at a suitable flow rate to hastendrying of the layer.

[0065] Optionally, during Step C, the layer cna be heated at suitabletemperatures from 200° C. to 700° C., preferably 200° C. to 400° C. tofix precious metal components within the composition. Preferably, theprecious metal component is fixed on a refractory oxide support. Thiscan be accomplished in the same manner as in the drying step except thatthe hot gas temperature is increased.

[0066] The coated, dried and coated honeycomb from Step C next goes toStep D where ambient temperature air is applied from 2 to 20 seconds andpreferably 5 to 20 seconds and preferably about 8 seconds in order tocool the coating as quickly as possible. This completes a coating stepfor a layer in the present invention. The ambient air is typically at atemperature range of from 5° to 40° C., of course other preferably inertgases can be used aside from air. Preferably, there is a hood such ashood 70 at the outlet 14′.

[0067] An additional coating can be provided through the inlet or bottomend 14. Alternatively, the honeycomb 10 can be rotated in Step E so thatthe outlet end 14′ becomes the bottom end and the inlet end 14 becomesthe top end to put coating through the outlet side. The process can berepeated to create a desired coating architecture on the coatedhoneycomb.

[0068] A specific embodiment of the method of the present inventioncomprises passing at least one inlet end aqueous liquid comprising aninlet end coating composition into a substrate, such as honeycomb 10 asrecited above. The aqueous liquid passes into the channel 16 inlets andextends for at least part of the length from the inlet end toward theoutlet end to form at least one inlet end layer coating such as layer 26shown in FIG. 3, with at least one inlet end coating extending 26 foronly part of the length from the inlet end 14 toward the outlet end 14′.The coating is dried by the application of a vacuum to the outlet endwhile forcing a gas stream through the channels from the inlet end afterthe formation of each inlet end coating without significantly changingthe length of each inlet layer coating.

[0069] The method includes passing at least one outlet end aqueousliquid comprising at least one outlet end coating composition into thesubstrate through the at least some of the channel outlets at thesubstrate outlet end. The aqueous liquid passes into the channels 16 andextending for at least part of the length from the outlet end 14′ towardthe inlet end 14 to form at least one outlet end layer coating 28 asshown in FIG. 3. Preferably, the method further comprises drying thecoating by applying a vacuum to the inlet end 14 while forcing a gasstream through the channels from the outlet end 14′ after the formationof each outlet end coating without significantly changing the length ofeach outlet layer coating.

[0070] Preferably, the method further comprises fixing the solublecomponents in the layer (e.g. 26) such as at least one precious metalcomponent selected from the first precious metal component of the inletlayer and the second precious metal component of the outlet layer tosaid at least one of the respective first or second component selectedfrom the first refractory oxide and first rare earth metal oxidecomponents, and the second refractory oxide and second rare earth metaloxide components, the fixing being conducted prior to coating the inletand outlet layers. The step of fixing can comprises chemically fixingthe precious metal on the respective refractory oxide and/or rare earthmetal oxide. More preferably, the step of fixing comprises thermallytreating the precious metal on the respective refractory oxide and/orrare earth metal oxide. The step of thermally treating the substrateupon completion of coating all layers at from 200° C. to 400° C. at from1 to 10, and preferably 2 to 6 seconds. The heat is provided by forcinga gas stream, preferably air which is heated to from 200° C. to 400° C.This temperature range has been found to substantially fix the solublecomponents such as precious metal components. The combination of flowrate and temperature of the gas stream should be sufficient to heat thecoating layer and preferably, providing a minimum of heat to theunderlying substrate to enable rapid cooling in the subsequent coolingstep prior to application of subsequent layers. Preferably, the steps ofthermally fixing each layer, preferably followed by cooling with ambientair, are conducted after coating and prior to coating a subsequentlayer. The cooling step is preferably conducted using ambient airtypically at from 5° C. to 40° C. at from 2 to 20, and preferably 4 to10 seconds. The combination of the ambient air flow rate and temperatureof the gas stream should be sufficient to cool the coating layer. Thismethod permits continuous coating of a plurality of layers on asubstrate to form the above described article of the present invention.

[0071] Following the step of fixing, there can be a step calcining theprecious metal on the respective refractory oxide and/or rare earthmetal oxide. This can be conducted between coating layers or morepreferably after the coating of all the layers on the substrate has beencompleted. The calcining can be conducted at from 250° C. to 900° C. atfrom 0.1 to 10 hours and preferably from 450° C. to 750° C. at from atfrom 0.5 to 2 hours. After the coating of all layers is complete thesubstrate can be calcined.

[0072] A preferred method comprises the step of fixing the at least oneprecious metal component selected from the first precious metalcomponent of the first layer and the second precious metal component ofthe second layer to said at least on to the respective first or secondcomponent selected from the first refractory oxide and first rare earthmetal oxide components, and the second refractory oxide and second rareearth metal oxide components, the fixing being conducted prior tocoating the first and second layers.

[0073] In yet another embodiment, the method comprises the step ofapplying a vacuum to the partially immersed substrate at an intensityand a time sufficient to draw the coating media upwardly to apredesignated distance to from the bath into each of the channels toform a uniform coating profile therein for each immersion step.Optionally, and preferably the substrate can be turned over to partiallyimmerse the substrate into the bath to coat with the second composition.The substrate should be thermally fixed after immersing the substrateinlet end, and after turning the substrate over and immersing the outletend.

[0074] In a preferred method of the present invention the substratecomprises an honeycomb monolith. The method comprises:

[0075] a) partially immersing the substrate into a vessel containing afirst coating composition, said vessel containing an amount of thecoating composition sufficient to coat the first zone of the substrate;

[0076] b) partially immersing the substrate into a vessel containing asecond coating composition, said vessel containing an amount of thecoating composition sufficient to coat the second zone of the substrate;

[0077] c) thermally treating at least the substrate after each immersionstep.

[0078] A vacuum can be applied to the partially immersed substrate at anintensity and a time sufficient to draw the coating media upwardly fromthe bath into each of the channels to form a uniform coating profiletherein for each immersion step.

[0079] The substrate can be turned over to prior to partially immersingthe substrate into the bath to coat with the second composition. Thesubstrate can be thermally fixed after immersing the substrate inletend, turning the substrate over and immersing the outlet end. There canbe an uncoated portion of the channel between zones one and two.

[0080] The honeycomb substrates of the present invention areparticularly useful in catalytically treating motor vehicle exhaust gasstreams comprising gaseous hydrocarbons, nitrogen oxides and carbonmonoxide. Additionally, the substrates of the present invention areuseful to treat exhaust gas streams from motor vehicles containingparticulate subject matter in the dry form, such as soot or volatileorganic fractions, both of which are found in diesel engine exhaust gasstreams. Finally, the present invention is useful in applications whereozone may be present in a gas stream, such as in environmental airtreated prior to being directed into an aircraft or vehicle cabin or invarious processes known to treat the environment such as disclosed inU.S. Ser. No. 08/682,174.

[0081] As indicated, a particularly preferred use of the presentinvention is for the treatment of motor vehicle exhaust gas streampollutants. Catalysts necessary to treat such pollutants typically havea goal to convert multiple pollutants to harmless products.Additionally, such catalysts have to operate at different conditions andin different parts of the exhaust gas stream. For example, usefulcatalysts to treat gaseous hydrocarbons, nitrogen oxides and carbonmonoxides are known as three-way catalysts and are located at variousparts of the exhaust system. Such catalysts may be located close to theengine and are referred to as close coupled catalysts or may be locateddownstream of the engine, typically under the floor of the passengercompartment and referred to as underfloor catalysts. Such embodimentsare shown in FIGS. 10 and 11.

[0082] Reference is made to FIG. 10 which illustrates a particular andpreferred embodiment of the present invention. FIG. 10 shows a motorvehicle 110 having a gasoline engine 112. The gasoline engine 112 has anengine exhaust outlet 114. In typical and preferred embodiments, theengine exhaust outlet 114 communicates to an engine exhaust manifold 116through manifold inlet 118. A close-coupled catalyst is in closeproximity to the engine exhaust manifold outlet 119. The manifold outlet119 is connected and communicates with close-coupled catalyst 120through close-coupled catalyst inlet 122. The close-coupled catalyst 120is connected to and communicates with a downstream catalyst, such asunderfloor catalytic converter 124. The close-coupled catalyst has aclose-coupled catalyst outlet 126 which is connected to the underfloorcatalyst 124 through the close-coupled catalyst exhaust pipe 130 tounder floor catalyst inlet 128. The underfloor catalyst 124 is typicallyand preferably connected to muffler 132. In particular, the underfloorcatalyst outlet 134 is connected to the muffler inlet 136 throughunderfloor exhaust pipe 138. The muffler has a muffler outlet 139 whichis connected to tailpipe 140 having a tailpipe outlet 142 which opens tothe environment.

[0083]FIG. 11 shows a schematic drawing of the close-coupled catalyst120 in combination with underfloor catalyst 124. In this preferredembodiment, the close-coupled catalyst comprises a close-coupledhoneycomb support 144 on which is coated the close-coupled catalystcomposition. The underfloor catalyst 124 comprises an underfloorhoneycomb 146 on which is coated an three-way catalyst composition. Theclose-coupled catalyst honeycomb of FIG. 2 can be sealingly mounted inclose-coupled canister 152 which has close-coupled catalyst inlet 122and close-coupled catalyst outlet 126 connected by close-coupledcatalyst exhaust pipe 130 to the inlet 128 of three-way catalyst 124which is sealingly mounted in underfloor catalyst canister 154.Underfloor exhaust pipe 138 is connected to underfloor catalyst outlet134. Alternatively, the close coupled catalyst composition can be in anupstream zone of a close coupled catalyst support 144. A three-waycatalyst can be located on the same honeycomb support 144 in adownstream zone. This can reduce or eliminate catalyst in the underfloorposition.

[0084] Any suitable substrate or carrier may be employed, such as amonolithic carrier of the type having a plurality of fine, parallel gasflow passages extending therethrough from an inlet or an outlet face ofthe carrier, so that the passages are open to fluid flow therethrough.The passages, which are essentially straight from their fluid inlet totheir fluid outlet, are defined by walls on which the catalytic materialis coated as a “washcoat” so that the gases flowing through the passagescontact the catalytic material. The flow passages of the monolithiccarrier are thin-walled channels which can be of any suitablecross-sectional shape and size such as trapezoidal, rectangular, square,sinusoidal, hexagonal, oval, circular. Such structures may contain fromabout 60 to about 1200 or more gas inlet openings (“cells”) per squareinch of cross section. The ceramic carrier may be made of any suitablerefractory material, for example, cordierite, cordierite-alpha alumina,silicon nitride, zircon mullite, spodumene, alumina-silica magnesia,zircon silicate, sillimanite, magnesium silicates, zircon, petalite,alpha alumina and aluminosilicates. The metallic honeycomb may be madeof a refractory metal such as a stainless steel or other suitable ironbased corrosion resistant alloys.

[0085] The carriers useful for the catalysts made by this invention maybe metallic in nature and be composed of one or more metals or metalalloys. The metallic carriers may be in various shapes such as pelletsor in monolithic form. Preferred metallic supports include theheat-resistant, base-metal alloys, especially those in which iron is asubstantial or major component. Such alloys may contain one or more ofnickel, chromium, and aluminum, and the total of these metals mayadvantageously comprise at least about 15 weight percent of the alloy,for instance, about 10 to 25 weight percent of chromium, about 3 to 8weight percent of aluminum and up to about 20 weight percent of nickel,say at least about 1 weight percent of nickel, if any or more than atrace amount be present. The preferred alloys may contain small or traceamounts of one or more other metals such as manganese, copper, vanadium,titanium and the like. The surfaces of the metal carriers may beoxidized at quite elevated temperatures, e.g., at least about 1000° C.,to improve the corrosion resistance of the alloy by forming an oxidelayer on the surface of carrier which is greater in thickness and ofhigher surface area than that resulting from ambient temperatureoxidation. The provision of the oxidized or extended surface on thealloy carrier by high temperature oxidation may enhance the adherence ofthe refractory oxide support and catalytically-promoting metalcomponents to the carrier.

[0086] Such monolithic carriers may contain up to about 1200 or moreflow channels (“cells”) per square inch of cross section, although farfewer may be used. For example, the carrier may have from about 60 to800, more usually from about 200 to 600, cells per square inch (“cpsi”)

[0087] Discrete layers of catalytic material, conventionally referred toas “washcoats”, can be coated onto a suitable carrier. Where there aremore than one layer in a given zone, e.g., two layers, preferably, thefirst coat adheres to the carrier and the second coat overlays andadheres to the first coat. With this arrangement, the gas beingcontacted with the catalyst, e.g., being flowed through the passagewaysof the catalytic material-coated carrier, will first contact the secondor top coat and pass therethrough in order to contact the underlyingbottom or first coat.

[0088] Preferred catalysts and catalyst structures are disclosed in WO95/35152, WO 95/00235 and WO 96/17671 hereby incorporated by reference.

[0089] When the compositions are applied as a thin coating to amonolithic carrier substrate, the proportions of ingredients areconventionally expressed as grams of material per cubic inch of catalystas this measure accommodates different gas flow passage cell sizes indifferent monolithic carrier substrates. The concentration of preciousmetal components such as platinum group metal components are based onthe weight of the platinum group metal and typically expressed in gramsof material per cubic foot.

[0090] In accordance with the present invention the catalyst can be inthe form of a catalyst composition supported on a substrate such as aceramic or metal monolith. The catalyst can be a coating on thesubstrate of one or more catalyst composition layers. A preferredcatalyst useful with the system of the present invention is a three-wayconversion catalyst (TWC). The TWC catalyst composite of the presentinvention simultaneously catalyzes the oxidation of hydrocarbons andcarbon monoxide and the reduction of nitrogen oxides in a gas stream.

[0091] Such compositions typically comprise a catalytically activecomponent. A useful and preferred component is a precious metal,preferably a platinum group metal and a support for the precious metal.Preferred supports are refractory oxides such as alumina, silica,titania, and zirconia. A catalyst system useful with the method andapparatus of the present invention comprises at least one substratecomprising a catalyst composition located thereon. The compositioncomprises a catalytically active material, a support and preferably anoxygen storage component.

[0092] Useful catalytically active components include at least one ofpalladium, platinum, rhodium, ruthenium, and iridium components, withplatinum, palladium and/or rhodium preferred. Precious metals aretypically used in amounts of up to 300 g/ft³, preferably 5 to 250 g/ft³and more preferably 25 to 200 g/³ ft depending on the metal. Amounts ofmaterials are based on weight divided by substrate (honeycomb) volume.

[0093] Useful supports can be made of a high surface area refractoryoxide support. Useful high surface area supports include one or morerefractory oxides selected from alumina, titania, silica and zirconia.These oxides include, for example, silica and metal oxides such asalumina, including mixed oxide forms such as silica-alumina,aluminosilicates which may be amorphous or crystalline,alumina-zirconia, alumina-chromia, alumina-ceria and the like. Thesupport is substantially comprised of alumina which preferably includesthe members of the gamma or activated alumina family, such as gamma andeta aluminas, and, if present, a minor amount of other refractory oxide,e.g., about up to 20 weight percent. Desirably, the active alumina has aspecific surface area of 60 to 300 m²/g.

[0094] Preferred oxygen storage components have oxygen storage andrelease capabilities. The oxygen storage component is any such materialknown in the art, preferably at least one oxide of a metal selected fromthe group consisting of rare earth metals, and most preferably a ceriumor praseodymium compound, with the most preferred oxygen storagecomponent being cerium oxide (ceria). The oxygen storage component canbe present at least 5 wt. % and preferably at least 10 wt. % and morepreferably at least 15 wt. % of the catalyst composition. The oxygenstorage component can be included by dispersing methods known in theart. Such methods can include impregnation onto the composition byimpregnating the oxygen storage component onto the a support such as apalladium containing support in the form of an aqueous solution, dryingand calcining the resulted mixture in air to give a first layer whichcontains an oxide of the oxygen storage component in intimate contactwith the palladium component. Examples of water soluble or dispersible,decomposable oxygen storage components which can be used include, butare not limited to water soluble salts and/or colloidal dispersions of,cerium acetate, praseodymium acetate, cerium nitrate, praseodymiumnitrate, etc. U.S. Pat. No. 4,189,404 discloses the impregnation ofalumina-based support composition with cerium nitrate.

[0095] Alternatively, the oxygen storage component can be a bulk oxygenstorage composition comprising an oxygen storage component which ispreferably ceria, and/or praseodymia in bulk form. Ceria is mostpreferred. By bulk form it is meant that the ceria and/or praseodymia ispresent as discrete particles which may be as small as 1 to 15 micronsin diameter or smaller, as opposed to having been dispersed in solutionas in the first layer. A description and the use of such bulk componentsis presented in U.S. Pat. No. 4,714,694, hereby incorporated byreference. As noted in U.S. Pat. No. 4,727,052, also incorporated byreference, bulk form means that particles of ceria are admixed withparticles of activated alumina so that the ceria is present in solid orbulk form as opposed to, for example, impregnating alumina particleswith a solution of ceria compound which upon calcination is converted toceria disposed within the alumina particles. Cerium oxide andpraseodymium oxide are the most preferred oxygen storage components.

[0096] The performance of the catalyst composition can be enhanced bythe use of an alkaline earth metal which is believed to act as astabilizer, at least one rare earth metal component selected fromlanthanum, praseodymium and neodymium which is believed to act as apromoter, and at least one zirconium component.

[0097] A useful and preferred catalyzed article can be a layered TWCcatalyst composite comprises a first (bottom) layer comprising a firstlayer composition and the second (top) layer comprising a second layercomposition. This composite contains palladium in both the first andsecond layer and in specific embodiments can comprise palladium assubstantially the only precious metal. Such articles are disclosed in WO95/00235.

[0098] Briefly, the first layer comprises a first platinum group metalcomponent, which comprises a first palladium component, which can be thesame or different than that in the second layer. For the first layer toresult in higher temperature conversion efficiencies, an oxygen storagecomponent is used in intimate contact with the platinum group metal. Itis preferred to use an alkaline earth metal component believed to act asa stabilizer, a rare earth metal selected from lanthanum and neodymiummetal components which is believed to act as a promoter, and a zirconiumcomponent. The second layer comprises a second palladium component andoptionally, at least one second platinum group metal component otherthan palladium. Preferably the second layer additionally comprises asecond zirconium component, at least one second alkaline earth metalcomponent, and at least one second rare earth metal component selectedfrom the group consisting of lanthanum metal components and neodymiummetal components. Preferably, each layer contains a zirconium component,at least one of the alkaline earth metal components and the rare earthcomponent. Most preferably, each layer includes both at least onealkaline earth metal component and at least one rare earth component.The first layer optionally further comprises a second oxygen storagecomposition which comprises a second oxygen storage component. Thesecond oxygen storage component and/or the second oxygen storagecomposition are preferably in bulk form and also in intimate contactwith the first platinum group metal component.

[0099] In a preferred embodiment the first layer can comprise a firstpalladium component and relatively minor amounts of a first platinumgroup metal other than palladium and/or the second layer can comprise asecond palladium component and relatively minor amounts of a secondplatinum group metal component other than a palladium component. Thepreferred first and second platinum group components are selected fromplatinum, rhodium, and mixtures thereof. The preferred first platinumgroup metal component other than palladium is platinum and the mostpreferred second platinum group metal component other than palladium isselected from rhodium, platinum, and mixtures thereof. Typically thefirst layer will contain up to 100 percent by weight of palladium as theplatinum group metal. Where a first platinum group metal component otherthan palladium is used, it is used typically in amounts up to 40 andpreferably from 0.1 to 40, more preferably from 5 to 25 percent byweight based on the total weight of the first palladium component andthe platinum group metal components other than palladium in the firstlayer. Where a second platinum group metal component other palladium isused, it is used typically in amounts up to 40 and preferably from 0.1to 40, more preferably from 5 to 25 percent by weight based on the totalweight of the second palladium component and the platinum group metalcomponents other than palladium in the second layer.

[0100] The catalyst of this embodiment preferably comprises a palladiumcomponent present in each of the first and second layers, in thecatalytically-active, promoting component in an amount sufficient toprovide compositions having significantly enhanced catalytic activitydue to the palladium component. In a preferred embodiment the firstpalladium component is the only platinum group metal component in thefirst layer, and the second palladium component is the only platinumgroup metal component in the second layer. Optionally either or both ofthe first and second layers can further respectively comprise a firstand second useful platinum group metals include, for instance, platinum,ruthenium, iridium and rhodium, and mixtures or alloys of such metals,e.g., platinum-rhodium.

[0101] The first layer composition and second layer compositionrespectively comprise a first support and a second support which can bethe same or different components. The support is made of a high surfacearea refractory oxide support as recited above. The first layer andsecond layer compositions preferably comprise a support such as alumina,catalytic components, stabilizers, reaction promoters and, if present,other modifiers and excludes the carrier or substrate. When thecompositions are applied as a thin coating to a monolithic carriersubstrate, the proportions of ingredients are conventionally expressedas grams of material per cubic inch of catalyst as this measureaccommodates different gas flow passage cell sizes in differentmonolithic carrier substrates. For typical automotive exhaust gascatalytic converters, the catalyst composite which includes a monolithicsubstrate generally may comprise from about 0.50 to about 6.0,preferablyabout 1.0 to about 5.0 g/in³ of catalytic composition coating.

[0102] The catalyst preferably contains a first oxygen storagecomponent, as recited above, in the first or bottom layer which is inintimate contact with a palladium component. The oxygen storagecomponent is any such material known in the art and preferably at leastone oxide of a metal selected from the group consisting of rare earthmetals and most preferably a cerium or praseodymium compound with themost preferred oxygen storage component being cerium oxide (ceria). Theoxygen storage component can be present at least 5 wt. % and preferablyat least 10 wt. % and more preferably at least 15 wt. % of the firstlayer composition. In the composition of the first or bottom layer, theoxygen storage component can be included by dispersing methods known inthe art such as by impregnating the oxygen storage component onto thepalladium containing support in the form of an aqueous solution, dryingand calcining the resulted mixture in air.

[0103] In the first or bottom layer, and in the top or second layerthere is optionally a first bulk oxygen storage composition comprisingan oxygen storage component which is preferably ceria, and/orpraseodymia in bulk form as recited. By bulk form it is meant that acomposition is in a solid, preferably fine particulate form, morepreferably having a particle size distribution such that at least about95% by weight of the particles typically have a diameter of from 0.1 to5.0, and preferably from 0.5 to 3 micrometers. Reference to thediscussion of bulk particles is made to U.S. Pat. No. 5,057,483 bothhereby incorporated by reference.

[0104] In addition to the above listed components of the first layercomposition and the second layer composition, it is optional that eachlayer contain a particular composite of zirconia and at least one rareearth oxide containing ceria. Such materials are disclosed for examplein U.S. Pat. Nos. 4,624,940 and 5,057,483, hereby incorporated byreference. Particularly preferred are particles comprising greater than50% of a zirconia-based compound and preferably from 60 to 90% ofzirconia, from 10 to 30 wt. % of ceria and optionally up to 10 wt. %,and when used at least 0.1 wt. %, of a non-ceria rare earth oxide usefulto stabilize the zirconia selected from the group consisting oflanthana, neodymia and yttria.

[0105] Both the first layer composition and second layer compositioncomprise a component which impart stabilization, preferably a firststabilizer in the first layer and second stabilizer in the second layer.The stabilizer is selected from the group consisting of alkaline earthmetal compounds. Preferred compounds include compounds derived frommetals selected from the group consisting of magnesium, barium, calciumand strontium. It is known from U.S. Pat. No. 4,727,052 that supportmaterials, such as activated alumina, can be thermally stabilized toretard undesirable alumina phase transformations from gamma to alpha atelevated temperatures by the use of stabilizers or a combination ofstabilizers. While a variety of stabilizers are disclosed, the firstlayer and second layer composition of the present invention use alkalineearth metal components. The alkaline earth metal components arepreferably alkaline earth metal oxide. In a particularly preferredcomposition, it is desirable to use barium and strontium as the compoundin the first and/or the second layer composition. The alkaline earthmetal can be applied in a soluble form which upon calcining becomes theoxide. It is preferred that the soluble barium be provided as bariumnitrate, barium acetate or barium hydroxide and the soluble strontiumprovided as strontium nitrate or strontium acetate, all of which uponcalcining become the oxides.

[0106] In each of the first layer and second layer compositions, theamount of metal oxide thermal stabilizer combined with the alumina maybe from about 0.05 to 30 weight percent, preferably from about 0.1 to 25weight percent, based on the total weight of the combined alumina,stabilizer and catalytic metal component.

[0107] Additionally, both the first layer composition and the secondlayer composition contain a compound derived from zirconium, preferablyzirconium oxide. The zirconium compound can be provided as a watersoluble compound such as zirconium acetate or as a relatively insolublecompound such as zirconium hydroxide. There should be an amountsufficient to enhance the stabilization and promotion of the respectivecompositions.

[0108] Both the first layer composition and the second layer compositioncontain at least one first promoter selected from the group consistingof lanthanum metal components and neodymium metal components with thepreferred components being lanthanum oxide (lanthana) and neodymiumoxide (neodymia). In a particularly preferred composition, there islanthana and optionally a minor amount of neodymia in the bottom layer,and neodymia or optionally lanthana in the top coat. While thesecompounds are known to act as stabilizers for the alumina support, theirprimary purpose in the composition of the present invention is to act asreaction promoters for the respective first and second layercompositions. A promoter is considered to be a material which enhancesthe conversion of a desired chemical to another. In a TWC the promoterenhances the catalytic conversion of carbon monoxide and hydrocarbonsinto water and carbon dioxide and nitrogen oxides into nitrogen andoxygen.

[0109] The first layer composition and/or the second layer compositionof the present invention can contain other conventional additives suchas sulfide suppressants, e.g., nickel or iron components. If nickeloxide is used, an amount from about 1 to 25% by weight of the first coatcan be effective. As disclosed in U.S. Pat. No. 5,057,483 herebyincorporated by reference.

[0110] A particularly useful layered catalyst composite of the presentinvention comprises in the first layer from about 0.003 to 0.3 g/in³ ofthe first palladium component; from about 0 to 0.065 g/in³ of the firstplatinum group metal component other than palladium; from about 0.15 toabout 2.0 g./in³ of the first support, i.e., alumina; at least about0.05 g/in³ of the total first oxygen storage component in intimatecontact with the palladium component; from about 0.025 to about 0.5g/in³ of at least one first alkaline earth metal components; from about0.025 to about 0.5 g/in³ of the first zirconium component; from about0.025 to about 0.5 g/in³ of at least one first rare earth metalcomponent selected from the group consisting of lanthanum metalcomponents and neodymium metal components; and comprises in the secondlayer from about 0.003 to 0.3 g/in³ of the second palladium componentand from about 0 to 0.065 g/in³ of a second rhodium component or asecond platinum component or mixture thereof, from about 0.15 g/in³ toabout 2.0 g/in³ of the second support, i.e., alumina; and from about0.025 to about 0.5 g/in³ of the second zirconium component. This firstand/or second layers can further comprise from about 0.025 g/in³ toabout 0.5 g/in³ of a nickel component. The first and/or second layersfurther can include the particulate composite of zirconia and ceria inamounts from 0.0 to 2.0 g/in³ comprising 60 to 90 wt. % zirconia, 10 to30 wt. % ceria and from 0 to 10 wt % rare earth oxides comprisinglanthana, neodymia and mixtures thereof. Weight of the palladiumcomponent and other platinum group metal components are based on theweight of the metal.

[0111] A useful and preferred first layer has:

[0112] from about 0.003 to about 0.6 g/in³ of at least one palladiumcomponent;

[0113] from 0 to about 0.065 g/in³ of at least one first platinum and/orfirst rhodium component;

[0114] from about 0.15 to about 2.0 g/in³ of a first support;

[0115] from about 0.05 to about 2.0 g/in³ of the total of the firstoxygen storage components in the first layer;

[0116] from 0.0 and preferably about 0.025 to about 0.5 g/in³ of atleast one first alkaline earth metal component;

[0117] from 0.0 and preferably about 0.025 to about 0.5 g/in³ of a firstzirconium component; and

[0118] from 0.0 and preferably about 0.025 to about 0.5 g/in³ of atleast one first rare earth metal component selected from the groupconsisting of ceria metal components, lanthanum metal components andneodymium metal component.

[0119] A useful and preferred second layer has:

[0120] from about 0.003 g/in³ to about 0.6 g/in³ of at least one secondpalladium component;

[0121] from 0.0 g/in³ to about 0.065 g/in³ of at least one firstplatinum and/or rhodium component;

[0122] from about 0.15 g/in³ to about 2.0 g/in ³ of a second support;

[0123] from 0.0 and preferably about 0.025 g/in³ to about 0.5 g/in³ ofat least one second rare earth metal component selected from the groupconsisting of lanthanum metal components and neodymium metal components;

[0124] from 0.0 and preferably about 0.25 g/in³ to about 0.5 g/in³ of atleast one second alkaline earth metal component; and

[0125] from 0.0 and preferably about 0.025 to about 0.5 g/in³ of asecond zirconium component. However, the first layer requires analkaline earth metal component and/or a rare earth component, and thesecond layer requires an alkaline earth metal component and/or a rareearth metal component.

[0126] The first and/or second layer can have from 0.0 to about 2.0g/in³ of an oxygen storage composite comprising particulate form ofcera-zirconia composite.

[0127] The discrete form and second coats of catalytic material,conventionally referred to as “washcoats”, are coated onto a suitablecarrier with, preferably, the first coat adhered to the carrier and thesecond coat overlying and adhering to the first coat are provided in onezone. With this arrangement, the gas being contacted with the catalyst,e.g., being flowed through the passageways of the catalyticmaterial-coated carrier, will first contact the second or top coat andpass therethrough in order to contact the underlying bottom or firstcoat. However, in an alternative configuration, the second coat need notoverlie the first coat but may be provided in an upstream first zone (assensed in the direction of gas flow through the catalyst composition)portion of the carrier, with the first coat provided on a downstreamsecond zone portion of the carrier. Thus, to apply the washcoat in thisconfiguration, an upstream first zone longitudinal segment only of thecarrier would be dipped into a slurry of the second coat catalyticmaterial, and dried, and the undipped downstream second zonelongitudinal segment of the carrier would then be dipped into a slurryof the first coat catalytic material and dried.

[0128] An alternative and useful TWC catalyst can contain more than oneprecious metal such as disclosed in WO 95/35152. The disclosed catalystof WO 95/35152 comprises a first layer comprising at least one firstpalladium component. The first layer can optionally contain minoramounts of a platinum component based on the total platinum metal of theplatinum components in the first and second layers. The second layercomprises at least two second platinum group metal components with oneof the platinum group metal components preferably being a platinumcomponent and the other preferably being a rhodium component.

[0129] Platinum group metal component support components in the firstand second layers can be the same or different and are preferablycompounds selected from the group consisting of silica, alumina andtitania compounds. Preferred first and second supports can be activatedcompounds selected from the group consisting of alumina, silica,silica-alumina, alumino-silicates, alumina-zirconia, alumina-chromia,and alumina-ceria.

[0130] A specific and preferred embodiment of the present inventionrelates to a layered catalyst composite comprising a first inner layerwhich comprises a first support having at least one palladium componentand from 0 to less than fifty weight percent based on platinum metal ofat least one first layer platinum component based on the total amount ofplatinum metal in the first and second layers.

[0131] Preferably, the first layer comprises a first support, a firstpalladium component, at least one first stabilizer, and at least onefirst rare earth metal component selected from ceria, neodymia andlanthana. The first layer can also comprise a first oxygen storagecomposition which comprises a first oxygen storage component. The secondlayer preferably comprises a second support, at least one secondplatinum component, at least one rhodium component, and a second oxygenstorage composition. There can be from fifty to one hundred weightpercent based on platinum metal of the second layer platinum componentbased on the total amount of platinum metal in the first and secondlayers.

[0132] The second layer preferably comprises a “second” oxygen storagecomposition which comprises a diluted second oxygen storage component.The oxygen storage composition comprises a diluent in addition to theoxygen storage component. Useful and preferred diluents includerefractory oxides. Diluent is used to mean that the second oxygenstorage component is present in the oxygen storage composition inrelatively minor amounts. The composition is a mixture which can becharacterized as a composite which may or may not be a true solidsolution. The second oxygen storage component is diluted to minimizeinteraction with the rhodium component. Such interaction may reduce longterm catalytic activity. The second layer preferably comprises a secondoxygen storage composition comprising a second oxygen storage componentsuch as rare earth oxide, preferably ceria. The second oxygen storagecomponent is diluted with a diluent such as a refractory metal oxide,preferably zirconia. A particularly preferred second oxygen storagecomposition is a co-precipitated ceria/zirconia composite. There ispreferably up to 30 weight percent ceria and at least 70 weight percentzirconia. Preferably, the oxygen storage composition comprises ceria,and one or more of lanthana, neodymia, yttria or mixtures thereof inaddition to ceria. A particularly preferred particulate compositecomprises ceria, neodymia and zirconia. Preferably there is from 60 to90 wt. % zirconia, 10-30% ceria and up to 10% neodymia. The ceria notonly stabilizes the zirconia by preventing it from undergoingundesirable phase transformation, but also behaves as an oxygen storagecomponent enhancing oxidation of carbon monoxide and the reduction ofnitric oxides.

[0133] Preferably, the second oxygen storage composition is in bulkform. By bulk form it is meant that the composition is in a solid,preferably fine particulate form, more preferably having a particle sizedistribution such that at least about 95% by weight of the particlestypically have a diameter of from 0.1 to 5.0, and preferably from 0.5 to3 micrometers. Reference to the discussion of bulk particles is made toU.S. Pat. Nos. 4,714,694 and 5,057,483 both hereby incorporated byreference.

[0134] The second oxygen storage component and optional first oxygenstorage component are preferably selected from the cerium group andpreferably consist of cerium compounds, praseodymium, and/or neodymiumcompounds. When using cerium group compounds it has been found that ifsulfur is present in the exhaust gas stream, objectionable hydrogensulfide can form. When it is preferred to minimize hydrogen sulfide, itis preferred to additionally use Group IIA metal oxides, preferablystrontium oxide and calcium oxide. Where it is desired to use cerium,praseodymium or neodymium compounds at least one of the first or secondlayers can further comprise a nickel or iron component to suppresshydrogen sulfide. Preferably, the first layer further comprises a nickelor iron component.

[0135] Stabilizers can be in either the first or second layers, and arepreferably in the first layer. Stabilizers can be selected from at leastone alkaline earth metal component derived from a metal selected fromthe group consisting of magnesium, barium, calcium and strontium,preferably strontium and barium.

[0136] Zirconium components in the first and/or second layers ispreferred and acts as both a stabilizer and a promoter. Rare earthoxides act to promote the catalytic activity of the first layercomposition. Rare earth metal components are preferably selected fromthe group consisting of lanthanum metal components and neodymium metalcomponents.

[0137] A useful and preferred first layer has:

[0138] from about 0.0175 to about 0.3 g/in³ of palladium component;

[0139] from about 0 to about 0.065 g/in³ of a first platinum component;

[0140] from about 0.15 to about 2.0 g/in³ of a first support;

[0141] from about 0.025 to about 0.5 g/in³ of at least one firstalkaline earth metal component;

[0142] from about 0.025 to about 0.5 g/in³ of a first zirconiumcomponent; and

[0143] from about 0.025 to about 0.5 g/in³ of at least one first rareearth metal component selected from the group consisting of ceria metalcomponents, lanthanum metal components and neodymium metal component.

[0144] A useful and preferred second layer has:

[0145] from about 0.001 g/in³ to about 0.03 g/in³ of a rhodiumcomponent;

[0146] from about 0.001 g/in³ to about 0.15 g/in³ of platinum;

[0147] from about 0.15 g/in³ to about 1.5 g/in³ of a second support;

[0148] from about 0.1 to 2.0 g/in³ of a second oxygen storagecomposition;

[0149] from about 0.025 g/in³ to about 0.5 g/in³ of at least one secondrare earth metal component selected from the group consisting oflanthanum metal components and neodymium metal components; and

[0150] from about 0.025 to about 0.5 g/in³ of a second zirconiumcomponent.

[0151] As above, the discrete form and second coats of catalyticmaterial, conventionally referred to as “washcoats”, are coated onto asuitable carrier with, preferably, the first coat adhered to the carrierand the second coat overlying and adhering to the first coat areprovided in one zone. With this arrangement, the gas being contactedwith the catalyst, e.g., being flowed through the passageways of thecatalytic material-coated carrier, will first contact the second or topcoat and pass therethrough in order to contact the underlying bottom orfirst coat. However, in an alternative configuration, the second coatneed not overlie the first coat but may be provided in an upstream firstzone (as sensed in the direction of gas flow through the catalystcomposition) portion of the carrier, with the first coat provided on adownstream second zone portion of the carrier. Thus, to apply thewashcoat in this configuration, an upstream first zone longitudinalsegment only of the carrier would be dipped into a slurry of the secondcoat catalytic material, and dried, and the undipped downstream secondzone longitudinal segment of the carrier would then be dipped into aslurry of the first coat catalytic material and dried.

[0152] The system of the present invention is also useful in combinationwith a stable close-coupled catalyst, a system comprising such aclose-coupled catalyst and a related method of operation as disclosed inWO 96/17671.

[0153] Close-coupled catalysts have been designed to reduce hydrocarbonemissions from gasoline engines during cold starts. More particularly,the close-coupled catalyst is designed to reduce pollutants inautomotive engine exhaust gas streams at temperatures as low as 350° C.,preferably as low as 300° C. and more preferably as low as 200° C. Theclose-coupled catalyst of the present invention comprises aclose-coupled catalyst composition which catalyzes low temperaturereactions. This is indicated by the light-off temperature. The light-offtemperature for a specific component is the temperature at which 50% ofthat component reacts.

[0154] The close-coupled catalyst is placed close to an engine to enableit to reach reaction temperatures as soon as possible. However, duringsteady state operation of the engine, the proximity of the close-coupledcatalyst to the engine, typically less than one foot, more typicallyless than six inches and commonly attached directly to the outlet of theexhaust manifold exposes the close-coupled catalyst composition toexhaust gases at very high temperatures of up to 1100° C. Theclose-coupled catalyst in the catalyst bed is heated to high temperatureby heat from both the hot exhaust gas and by heat generated by thecombustion of hydrocarbons and carbon monoxide present in the exhaustgas. In addition to being very reactive at low temperatures, theclose-coupled catalyst composition should be stable at high temperaturesduring the operating life of the engine. A catalyst downstream of theclose-coupled catalyst can be an underfloor catalyst or a downstreamcatalyst. As recited above a TWC catalyst can be located on the closecoupled honeycomb 144 in a zone downstream of an upstream zone whichcomprises the close coupled catalyst composition. When the underfloorcatalyst is heated to a high enough temperature to reduce thepollutants, the reduced conversion of carbon monoxide in theclose-coupled catalyst results in a cooler close-coupled catalyst andenables the downstream catalyst typically the underfloor three-waycatalyst to burn the carbon monoxide and run more effectively at ahigher temperature. The downstream or underfloor catalyst preferablycomprises an oxygen storage component as described above.

[0155] The close-coupled catalyst preferably is in the form of a carriersupported catalyst where the carrier comprises a honeycomb type carrier.A preferred honeycomb type carrier comprises a composition having atleast about 50 grams per cubic foot of palladium component, from 0.5 to3.5 g/in³ of activated alumina, and from 0.05 to 0.5 g/in³ of at leastone alkaline earth metal component, most preferably, strontium oxide.Where lanthanum and/or neodymium oxide are present, they are present inamounts up to 0.6 g/in³.

[0156] The close coupled catalyst, in one or more layer can be used in afirst upstream zone. Preferably, a TWC catalyst can be used in one ormore downstream zones.

[0157] The aqueous coating compositions useful for the present inventioncan be made by adding a finely-divided, high surface area, refractoryoxide support to a solution of a water-soluble, catalytically-promotingmetal component, preferably containing one or more platinum group metalcomponents to form a slurry typically having from 20 to 40 weightpercent solids. Other additives including stabilizers, oxygen storagecomponents and the like can also be added at this point. Slurries madeaccording to this method using the compositions recited above can beused as the coating compositions in accordance with the method of thepresent invention.

[0158] In making catalysts by this invention, the catalytically-activecomposite of the fixed or water-insoluble catalytically-promoting metalcomponent and high area support can be coated on the substrate. This canbe accomplished by first comminuting the catalytically-active compositeor plurality of such composites, as an aqueous slurry which ispreferably acidic. This treatment is usually continued until the solidparticles in the slurry have particle sizes which are mostly below about10 or 15 micrometers. The comminution can be accomplished in a ball millor other suitable equipment, and the solids content of the slurry my be,for instance, about 20 to 50 weight percent, preferably about 35 to 45weight percent. The pH of the slurry is preferably below about 5 andacidity may be supplied by the use of a minor amount of a water-solubleorganic or inorganic acid or other water-soluble acidic compounds. Thusthe acid employed may be hydrochloric or nitric acid, or more preferablya lower fatty acid such as acetic acid, which may be substituted with,for example, chlorine as in the case of trichloroacetic acid. The use offatty acids may serve to minimize any loss of platinum group metal fromthe support.

[0159] Each layer of the present composite can also be prepared by themethod in disclosed in U.S. Pat. No. 4,134,860 (incorporated byreference) generally recited as follows.

[0160] A finely-divided, high surface area, refractory oxide support iscontacted with a solution of a water-soluble, catalytically-promotingmetal component, preferably containing one or more platinum group metalcomponents, to provide a mixture which is essentially devoid of free orunabsorbed liquid. The catalytically-promoting platinum group metalcomponent of the solid, finely-divided mixture can be converted at thispoint in the process into an essentially water-insoluble form while themixture remains essentially free of unabsorbed liquid. This process canbe accomplished by employing a refractory oxide support, e.g., alumina,including stabilized aluminas, which is sufficiently dry to absorbessentially all of the solution containing the catalytically-promotingmetal component, i.e., the amounts of the solution and the support, aswell as the moisture content of the latter, are such that their mixturehas an essential absence of free or unabsorbed solution when theaddition of the catalytically-promoting metal component is complete.During the latter conversion or fixing of the catalytically-promotingmetal component on the support, the composite remains essentially dry,i.e., it has substantially no separate or free liquid phase.

[0161] The mixture containing the fixed, catalytically-promoting metalcomponent can be comminuted as a slurry which is preferably acidic, toprovide solid particles that are advantageously primarily of a size ofup to about 5 to 15 microns. The resulting slurry is useful to coat thesubstrate 10, dried and preferably calcined.

[0162] Precious metal group or base metal group components, alone or inmixtures, may be formed in separate first and second layers on thesubstrate. If the metal components are not selectively deposited on thecarrier and fixed to the refractory oxide, they may move freely from onelayer of the catalyst to the next.

[0163] Alternatively, catalytically-promoting metal solution and higharea refractory oxide support can combined the catalytically-promotingmetal component can be fixed on the support, i.e., converted toessentially water-insoluble form, while the composite remainsessentially devoid of free or unabsorbed aqueous medium. The conversionmay be effected chemically, by treatment with a gas such as hydrogensulfide or hydrogen or with a liquid such as acetic acid or other agentswhich may be in liquid form, especially an aqueous solution, e.g.,hydrazine. The amount of liquid used, however, is not sufficient for thecomposite to contain any significant or substantial amount of free orunabsorbed liquid during the fixing of the catalytically-promoting metalon the support. The fixing treatment may be with a reactive gas or onewhich is essentially inert; for example, the fixing may be accomplishedby calcining the composite in air or other gas which may be reactivewith the catalytically-promoting metal component or essentially inert.The resulting insoluble or fixed catalytically-promoting metal componentmay be present as a sulfide, oxide, elemental metal or in other forms.When a plurality of catalytically-promoting metal components aredeposited on a support, fixing may be employed after each metalcomponent deposition or after deposition of a plurality of such metalcomponents.

[0164] The particle size of the finely-divided, high surface area,refractory oxide support is generally above about 10 or 15 micrometers.As noted above, when combined with the catalytically-promotingmetal-containing solution the high area support is sufficiently dry toabsorb essentially all of the solution.

[0165] The comminuted catalytic composition can be deposited on thecarrier in a desired amount, for example, the composition may compriseabout 2 to 30 weight percent of the coated carrier, and is preferablyabout 5 to 20 weight percent. The composition deposited on the carrieris generally formed as a coating over most, if not all, of the surfacesof the carrier contacted.

[0166] The catalytic compositions made by the present invention can beemployed to promote chemical reactions, such as reductions, methanationsand especially the oxidation of carbonaceous materials, e.g., carbonmonoxide, hydrocarbons, oxygen-containing organic compounds, and thelike, to products having a higher weight percentage of oxygen permolecule such as intermediate oxidation products, carbon dioxide andwater, the latter two materials being relatively innocuous materialsfrom an air pollution standpoint. Advantageously, the catalyticcompositions can be used to provide removal from gaseous exhausteffluents of uncombusted or partially combusted carbonaceous fuelcomponents such as carbon monoxide, hydrocarbons, and intermediateoxidation products composed primarily of carbon, hydrogen and oxygen, ornitrogen oxides. Although some oxidation or reduction reactions mayoccur at relatively low temperatures, they are often conducted atelevated temperatures of, for instance, at least about 150° C.,preferably about 200° to 900° C., and generally with the feedstock inthe vapor phase. The materials which are subject to oxidation generallycontain carbon, and may, therefore, be termed carbonaceous, whether theyare organic or inorganic in nature. The catalysts are thus useful inpromoting the oxidation of hydrocarbons, oxygen-containing organiccomponents, and carbon monoxide, and the reduction of nitrogen oxides.These types of materials may be present in exhaust gases from thecombustion of carbonaceous fuels, and the catalysts are useful inpromoting the oxidation or reduction of materials in such effluents. Theexhaust from internal combustion engines operating on hydrocarbon fuels,as well as other waste gases, can be oxidized by contact with thecatalyst and molecular oxygen which may be present in the gas stream aspart of the effluent, or may be added as air or other desired formhaving a greater or lesser oxygen concentration. The products from theoxidation contain a greater weight ratio of oxygen to carbon than in thefeed material subjected to oxidation. Many such reaction systems areknown in the art.

What is claimed is:
 1. An article comprising: a substrate comprising aninlet end, an outlet end, axial wall elements extending from the inletend to the outlet end, and a plurality of axially enclosed channelsdefined by the the wall elements, with at least some of the channelshaving a channel inlet at the inlet end and a channel outlet at theoutlet end; a first inlet layer located on the walls and extending forat least part of the length from the inlet end toward the outlet end toan inlet layer axial end, with the first inlet layer extending for onlypart of the length from the inlet end toward the outlet end, the atleast one inlet layer comprising a first inlet composition comprising atleast one first inlet component selected from first inlet base metaloxides; the first inlet layer coated by a method comprising the stepsof: passing a fluid containing the first inlet composition into theinlet end of the substrate to form the first inlet layer; and applying avacuum to the outlet end while forcing a heated gas stream through thechannels from the inlet end without significantly changing the length ofthe first inlet layer.
 2. The article as recited in claim 1 wherein thefirst inlet base metal oxides are selected from a first inlet refractoryoxide, a first inlet rare earth metal oxide, a first inlet transitionmetal oxide, and a first inlet alkaline earth metal oxide, and a firstinlet molecular sieve.
 3. The article as recited in claim 1 wherein theheated gas is air.
 4. The article as recited in claim 1 wherein thetemperature of the heated gas is from 75° C. to 400° C.
 5. The articleas recited in claim 4 wherein the temperature of the heated gas is from75° C. to 200° C. to dry the inlet layer.
 6. The article as recited inclaim 1 further comprising at least one first inlet precious metalcomponent.
 7. The article as recited in claim 6 wherein the temperatureof the heated gas is from 75° C. to 400° C.
 8. The article as recited inclaim 7 wherein the temperature of the heated gas is from 200° C. to400° C. to fix the first inlet precious metal component.
 9. The articleas recited in claim 1 further comprising at least one second inlet layerlocated on the walls and extending for at least part of the length fromthe inlet end toward the outlet end to a second layer axial end, the atleast one second layer supported directly or indirectly on the firstinlet layer for at least part of the length of the first inlet layer,the at least one second layer comprising a second inlet compositioncomprising at least one second inlet component selected from secondinlet base metal oxides; the at least one second inlet layer coated by amethod comprising the steps of: passing a fluid containing the at leastone second inlet composition into the inlet end of the substrate to formthe at least one inlet layer; and applying a vacuum to the outlet endwhile forcing a heated gas stream through the channels from the inletend without significantly changing the length of the at least one secondinlet layer.
 10. The article as recited in claim 9 wherein the at leastone second inlet base metal oxides are selected from a second inletrefractory oxide, a second inlet rare earth metal oxide, a second inlettransition metal oxide, and a second inlet alkaline earth metal oxide,and a second inlet molecular sieve.
 11. The article as recited in claim9 wherein the heated gas is air.
 12. The article as recited in claim 9wherein the temperature of the heated gas is from 75° C. to 400° C. 13.The article as recited in claim 12 wherein the temperature of the heatedgas is from 75° C. to 200° C. to dry the inlet layer.
 14. The article asrecited in claim 9 further comprising at least one second inlet preciousmetal component.
 15. The article as recited in claim 14 wherein thetemperature of the heated gas is from 75° C. to 400° C.
 16. The articleas recited in claim 15 wherein the temperature of the heated gas is from200° C. to 400° C. to fix the precious metal component.
 17. The articleas recited in claim 16 wherein there is at least one precious metalcomponent selected from the first inlet precious metal component and thesecond inlet precious metal component.
 18. The article as recited inclaim 17 wherein there is at least one precious metal component selectedfrom the first inlet precious metal component and the second inletprecious metal component and said precious metal components are selectedfrom at least one of platinum, palladium, rhodium, ruthenium and iridiumcomponents.
 19. The article as recited in claim 1 further comprising: afirst outlet layer located on the walls and extending for at least partof the length from the outlet end toward the inlet end to an outletlayer axial end, with the first outlet layer extending for only part ofthe length from the outlet end toward the inlet end, the at least oneoutlet layer comprising a first outlet composition comprising at leastone first outlet component selected from first outlet base metal oxides;the first outlet layer coated by a method comprising the steps of:passing a fluid containing the first outlet composition into the outletend of the substrate to form the first outlet layer; and applying avacuum to the outlet end while forcing a heated gas stream through thechannels from the outlet end without significantly changing the lengthof the first outlet layer.
 20. The article as recited in claim 19wherein the first outlet base metal oxides are selected from a firstoutlet refractory oxide, a first outlet rare earth metal oxide, a firstoutlet transition metal oxide, and a first outlet alkaline earth metaloxide and first outlet molecular sieves.
 21. The article as recited inclaim 20 wherein the heated gas is air.
 22. The article as recited inclaim 19 wherein the temperature of the heated gas is from 75° C. to400° C.
 23. The article as recited in claim 22 wherein the temperatureof the heated gas is from 75° C. to 200° C. to dry the outlet layer. 24.The article as recited in claim 19 further comprising at least one firstoutlet precious metal component.
 25. The article as recited in claim 24wherein the temperature of the heated gas is from 75° C. to 400° C. 26.The article as recited in claim 25 wherein the temperature of the heatedgas is from 200° C. to 400° C. to fix the first outlet precious metalcomponent.
 27. The article as recited in claim 19 further comprising: atleast one second outlet layer located on the walls and extending for atleast part of the length from the outlet end toward the inlet end to asecond layer axial end, the at least one second layer supported directlyor indirectly on the first outlet layer for at least part of the lengthof the first outlet layer, the at least one second layer comprising asecond outlet composition comprising at least one second outletcomponent selected from second outlet base metal oxides; the at leastone second outlet layer coated by a method comprising the steps of:passing a fluid containing the at least one second outlet compositioninto the outlet end of the substrate to form the at least one outletlayer; and applying a vacuum to the outlet end while forcing a heatedgas stream through the channels from the outlet end withoutsignificantly changing the length of the at least one second outletlayer.
 28. The article as recited in claim 27 wherein the at least onesecond outlet base metal oxides are selected from a second outletrefractory oxide, a second outlet rare earth metal oxide, a secondoutlet transition metal oxide, and a second outlet alkaline earth metaloxide, and a second outlet molecular sieve.
 29. The article as recitedin claim 27 wherein the heated gas is air.
 30. The article as recited inclaim 27 wherein the temperature of the heated gas is from 75° C. to400° C.
 31. The article as recited in claim 30 wherein the temperatureof the heated gas is from 75° C. to 200° C. to dry the outlet layer. 32.The article as recited in claim 27 further comprising at least onesecond outlet precious metal component.
 33. The article as recited inclaim 32 wherein the temperature of the heated gas is from 75° C. to400° C.
 34. The article as recited in claim 33 wherein the temperatureof the heated gas is from 200° C. to 400° C. to fix the precious metalcomponent.
 35. The article as recited in claim 32 wherein there is atleast one precious metal component selected from the first outletprecious metal component and the second outlet precious metal component.36. The article as recited in claim 35 wherein there is at least oneprecious metal component selected from the first outlet precious metalcomponent and the second outlet precious metal component and saidprecious metal components are selected from at least one of platinum,palladium, rhodium, ruthenium and iridium components.
 37. The article asrecited in claims 19 or 27 wherein at least a portion of at least one ofthe first or second inlet layers over laps with at least one of thefirst or second outlet layers.
 38. The article as recited in claims 1,9, 19 or 27 wherein the substrate has at least two adjacent zones, afirst zone and a second zone, each extending axially along the length ofconduit wherein the first zone extends from the inlet and the secondzone extends from the outlet along a separate length of the conduit thanthe first zone with each zone comprising the same catalyst architecturewith said zone.
 39. The article as recited in claim 37 wherein the atleast one layer of said first zone, and at least one layer of saidsecond zone overlap to form at least one intermediate zone between thefirst zone and the second zone.
 40. The article as recited in claim 37wherein there is an uncoated zone between the first zone and the secondzone.
 41. The article as recited in claim 37 wherein there are at leastthree zones.
 42. The article as recited in claims 1, 9, 19 or 27 whereinthe substrate comprises a monolithic honeycomb comprising a plurality ofparallel channels extending from the inlet to the outlet.
 43. Thearticle as recited in claim 42 wherein the honeycomb is selected fromthe group comprising ceramic monoliths and metallic monoliths.
 44. Thearticle as recited in claim 42 wherein the honeycomb is selected fromthe group comprising flow through monoliths and wall flow monoliths. 45.The article as recited in claims 1, 9, 19 or 27 wherein at least onelayer contains no precious metal component.
 46. The article as recitedin claims 1, 9, 19 or 27 wherein the comprising at least one inlet layerand at least one outlet layer, at least inlet composition comprising atleast one first inlet refractory oxide composition or compositecomprising a first inlet refractory oxide selected from the groupconsisting of alumina, titania, zirconia and silica, an inlet andoptionally a zeolite, and at least one inlet precious metal component,and the at least one outlet layer comprising an outlet compositioncomprising at least one outlet refractory oxide composition or selectedfrom the group consisting of alumina, titania, zirconia and silica, andat least one second outlet precious metal component, and optionally anoutlet zeolite.
 47. The article as recited in claim 46 wherein the inletcompositions contain substantially no oxygen storage components.
 48. Thearticle as recited in claim 47 wherein the inlet compositions containsubstantially no oxygen storage components selected from praseodymiumand cerium components.
 49. The article as recited in claim 46 wherein atleast one of the outlet compositions contain an oxygen storagecomponents.
 50. The article as recited in claim 49 wherein at least oneof the outlet compositions contains an oxygen storage component selectedfrom praseodymium and cerium components.
 51. The article as recited inclaim 46 wherein the at least one inlet precious metal component isfixed to the at least one of the inlet refractory oxide composition orcomposite and the first rare earth metal oxide, and the at least one ofthe outlet precious metal component is fixed to the at least one of theoutlet refractory oxide composition or composite and the rare earthmetal oxide.
 52. A method comprising: passing at least one inlet endfluid comprising an inlet end coating composition into a substrate, thesubstrate comprising an inlet end, an outlet end, wall elementsextending between the inlet end to the outlet end and a plurality ofaxially enclosed channels defined by the wall elements, at least some ofthe channels having a channel inlet at the inlet end and a channeloutlet at the outlet end, the aqueous liquid passing into the channelinlets and extending for at least part of the length from the inlet endtoward the outlet end to form at least one inlet end layer coating, withat least one inlet end coating extending for only part of the lengthfrom the inlet end toward the outlet end; applying a vacuum to theoutlet end while forcing a gas stream through the channels from theinlet end after the formation of each inlet end coating withoutsignificantly changing the length of each inlet layer coating; passingat least one outlet end aqueous fluid comprising at least one outlet endcoating composition into the substrate through the at least some of thechannel outlets at the substrate outlet end, the aqueous liquid passinginto the channels and extending for at least part of the length from theoutlet end toward the inlet end to form at least one outlet end layercoating.
 53. The method as recited in claim 52 further comprisingapplying a vacuum to the inlet end while forcing a gas stream throughthe channels from the outlet end after the formation of each outlet endcoating without significantly changing the length of each outlet layercoating.
 54. The method as recited in claim 52 wherein at least oneoutlet end coating extends for only part of the length from the outletend toward the inlet end.
 55. The method as recited in claim 52 whereinat least one inlet layer comprises a first inlet composition comprisingat least one first inlet component selected from a first refractoryoxide and a first inlet rare earth metal oxide and optionally at leastone first inlet precious metal component.
 56. The method as recited inclaim 53 wherein at least one outlet layer comprises a first compositioncomprising at least one first outlet component selected from a firstrefractory oxide and a first outlet rare earth metal oxide andoptionally at least one first outlet precious metal component.
 57. Themethod as recited in claims 55 or 56 further comprising the step offixing the at least one precious metal component selected from the inletprecious metal component of the at least one inlet layer and the outletprecious metal component of the at least one outlet layer to said atleast one of the respective inlet or outlet component selected from theinlet refractory oxide and inlet rare earth metal oxide components, andthe outlet refractory oxide and outlet rare earth metal oxidecomponents, the fixing being conducted prior to coating the inlet andoutlet layers.
 58. The method as recited in claim 57 wherein the step offixing comprises chemically fixing the precious metal component on therespective refractory oxide and/or rare earth metal oxide.
 59. Themethod as recited in claim 58 wherein the step of fixing comprisesthermally treating the precious metal component on the respectiverefractory oxide and/or rare earth metal oxide.
 60. The method asrecited in claim 59 wherein the step of fixing comprises calcining theprecious metal component on the respective refractory oxide and/or rareearth metal oxide.
 61. The method as recited in claim 60 wherein thestep of calcining is conducted at from 250° C. to 900° C. at from 0.1 to10 hours.
 62. The method as recited in claims 52 or 53 furthercomprising the steps of thermally fixing each layer after coating andprior to coating a subsequent layer.
 63. The method as recited in claim62 further comprising the step of thermally treating the substrate uponcompletion of coating all layers at from 200° C. to 400° C. at from 1 to10 seconds.
 64. The method as recited in claim 52 further comprising thesteps of calcining the substrate upon completion of coating all layers.65. The method as recited in claim 64 wherein the step of calcining isconducted at from 250° C. to 900° C. at from 0.1 to 10 hours.
 66. Amethod for coating a substrate comprising an inlet end, an outlet end,axial wall elements extending from the inlet end to the outlet end and aplurality of axially enclosed channels defined by the wall elements,with at least some of the channels having a channel inlet at the inletend and a channel outlet at the outlet end, comprising the steps of: a)partially immersing the substrate at the inlet end into a vesselcontaining a first coating composition, at least once, to form at leastone first layer located on the walls and extending for at least part ofthe length from the inlet end toward the outlet end, with at least oneinlet end coating extending for only part of the length from the inletend toward the outlet end; b) partially immersing the substrate at theoutlet end into a vessel containing a second coating composition, atleast once, to form at least one second layer located on the walls andextending for at least part of the length from the outlet end toward theinlet end; and c) thermally treating the substrate after each immersionstep, to form at least two zones, a first zone extending from the inletend and a second zone, each extending along the channels wherein thesecond zone extends along a separate length of the channel than thefirst zone.
 67. The method as recited in claim 66 further comprising thesteps of thermally fixing each layer after coating and prior to coatinga subsequent layer.
 68. The method as recited in claim 66 furthercomprising the steps of thermally treating the substrate upon completionof coating all layers at from 200° C. to 400° C. at from 1 to 10seconds.
 69. The method as recited in claim 66 further comprising thestep of calcining the substrate.
 70. The method as recited in claim 69wherein the step of calcining is conducted at from 250° C. to 900° C. atfrom 0.1 to 10 hours.
 71. The method as recited in claim 66 furthercomprising the step of applying a vacuum to the partially immersedsubstrate at an intensity and a time sufficient to draw the coatingmedia upwardly from the bath into each of the channels to form a uniformcoating profile therein for each immersion step.
 72. The method asrecited in claim 66 comprising the step thermally fixing after immersingthe substrate inlet end, turning the substrate over and immersing theoutlet end.
 73. The method as recited in claim 66 wherein there is anuncoated portion of the channel between the first and second zones. 74.A method comprising the steps of: contacting a gas comprising nitrogenoxide, carbon monoxide and hydrocarbon with an article as recited inclaims 1 or 19.